The present invention relates generally to implantable medical devices for providing stimulating pulses to selected body tissue, and more particularly, to the lead assemblies connecting such devices with the tissue to be stimulated.
Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled electrical stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing, or for sensing electrical signals produced by the heart and representing cardiac activity, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end a tip electrode having an active tip surface designed to intimately contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end and the electrode at the distal end.
To prevent displacement or dislodgment of the tip electrode and to maintain the necessary stable electrical contact between the tip electrode and the endocardial tissue, the electrode must be firmly anchored relative to the tissue. A number of methods, both passive and active, have been devised for this purpose. In accordance with one known passive fixation technique, a plurality of flexible tines bonded to or molded integrally with the insulative sheath covering the coiled electrical conductors extend rearwardly at an acute angle relative to the longitudinal axis of the lead. Following implantation of the lead, the tines become anchored in the trabecular network of the heart thereby securing the electrode against displacement or dislodgment. In this fashion, the position of the tip electrode is mechanically stabilized, that is, the tip electrode is positively anchored so as to remain in place during the lifetime of the implant. Since the tines can flatten against the lead body and thus reduce its diameter, tined leads are often suitable for introduction through small blood veins. Other known passive fixation techniques include collar electrodes which have one or more conical projections of silicon rubber or other biostable, biocompatible flexible material behind the electrode tip surface. Like the tines, the conical projections become entangled in the trabecular network inside the heart thereby anchoring the tip electrode.
The tip electrode of an active fixation lead may comprise, in accordance with one form thereof, a pointed helix adapted to be screwed into the heart tissue to be stimulated. Rotational torque applied to the connector pin at the proximal end of the lead is transmitted via the flexible, coiled conductor to the helical electrode which is thereby screwed into the heart tissue. In this fashion, the position of the electrode tip is mechanically stabilized. Removal of the screw-in electrode from the endocardium can be effected by counter-rotation of the connector pin. Thus, in a rotatable pin, screw-in type, active fixation lead, the conductor coil is used not only as an electrical conductor coupling for the connector pin and the helix electrode, but also as a tool for extending or retracting the helix electrode relative to the distal tip of the lead during lead fixation by rotating the connector pin.
Occasionally a pacing lead may cause infection. If the infection is refractory to antibiotic therapy, the lead must be surgically removed. In other situations, a patient may receive several leads because of xe2x80x9cexit blockxe2x80x9d or because they have required pacing therapy since childhood. In these situations, it would be ideal if the superfluous leads were easily removed with little risk to the patient. One possibility is to provide a lead that has a uniform diameter and is made of a material that does not xe2x80x9cstickxe2x80x9d to tissue. In the case of passive leads, the fixation tines at the distal tip are typically encapsulated with fibrotic tissue and are difficult to extract. In fact, leads are sometimes so heavily fixated, they come apart during the extraction process. Fragments are often left behind, which is highly undesirable because it may entail a risk of reinfection and may also obstruct the insertion of a new pacing lead. Active fixation leads are less subject to explantation difficulties because the helix tip electrode typically does not resist unscrewing and removal. Nevertheless, on occasion fibrotic encapsulation makes removal of even an active fixation lead difficult with attendant risk to the patient. Thus, lead designs that facilitate explantation are of great value.
To avoid destruction of a pacing lead during extraction it is possible, for example, to use a tubular xe2x80x9ccut-loosexe2x80x9d catheter which fits outside and around the pacing lead and which can be advanced towards the tip electrode along the pacing lead. At its forward end the cut-loose catheter is provided with a cutting edge for cutting through the encapsulating fibrotic tissue, thereby releasing the pacing lead and permitting its extraction. Another useful method is to insert an extraction stylet in the central channel or lumen of the pacing lead. At its forward end this stylet has a protruding helix or screw which can be brought into engagement with the conductor coil of the pacing lead near the electrode tip, thereby making a withdrawal of the entire pacing lead possible without the risk of the electrode tip or a longer forward end section of the pacing lead breaking loose from the remaining part thereof.
The above-described methods, however, do not always work and may also expose the patient to certain risks, for example, the risk of causing tamponade.
Another approach to the problem of managing the removal of an implanted cardiac pacing lead is disclosed in U.S. Pat. No. 5,179,962. In that patent, a cardiac lead includes a distal end portion comprising a fixation assembly having fixation members in the form of extendable wire rods. These fixation members are movable between an inactive retracted position wherein they are completely retracted into the fixation assembly, and an active extended position wherein they protrude obliquely backwards from the outer surface of the assembly. The fixation members protrude in a barb-like manner from a region of the outer surface of the fixation assembly which is located a short distance behind an annular endocardium contact surface at the distal end of the assembly. The adjustment of the barb-like fixation members between their inactive and active positions takes place by means of an elongated stylet which is axially displaced within the cardiac lead and fixation assembly. The front or distal end of this stylet is attached to a retainer which is displaced in a piston-like manner within the fixation assembly. The fixation members have their front ends attached to said retainer. The fixation members extend obliquely backwards from the rear end of the retainer and protrude obliquely backwards through openings in a jacket or sleeve which encloses the fixation assembly on the outside thereof. This jacket or sleeve is made of an electrically insulating material. However, this prior art cardiac lead with retractable/extendable fixation members is complex and therefore difficult and expensive to manufacture.
Yet another implantable pacing system lead including a detachably connected tip electrode is disclosed in European patent application No. 041254 published Dec. 9, 1981. The lead disclosed in this application includes a connector having a distal threaded end received by an internally threaded sleeve formed integrally with the tip electrode. The proximal end of the connector has a bore for receiving a coil conductor. The coil conductor is fixed to the wall of the bore by means of a longitudinal pin disposed within the lumen of the conductor. The diameter of the pin is such that the pin forces the coil conductor into secure engagement with the wall of the connector bore. The pin includes at its proximal end a profiled section in the form of a groove for receiving a correspondingly shaped distal end of a removal stylet. Rotation of the removal stylet unscrews the connector from the tip electrode permitting withdrawal of the lead body, leaving behind the implanted tip electrode. The disadvantages of this arrangement is that rotation of the removal stylet relative to the lead body can cause rotation of the tip electrode with consequent tissue damage, risk of infection, and so forth.
The present invention provides a pacing lead assembly including a lead body that is separable from the tip electrode assembly when the lead fixation means, such as tines, are encapsulated with fibrotic tissue and therefore difficult to extract. Preferably, the lead body is isodiametric and is coated with a xe2x80x9cnon-stickingxe2x80x9d material like Teflon(copyright). In the preferred embodiment, the tip electrode assembly includes a proximal end having a threaded bore while the isodiametric lead body has a screw element received by the threaded bore in the tip electrode assembly. The tip electrode assembly may be separated from the lead body by using a stylet having a specially shaped distal end that can engage a correspondingly shaped socket or channel in the proximal end of the threaded element. Holding the lead body stationary, that is, against rotation, the stylet is rotated in a direction to unscrew the threaded element from the tip electrode assembly thereby disconnecting the lead body therefrom. This leaves the small tip electrode assembly behind in the fibrotic tissue. Since the lead body is isodiametric and is coated with a non-sticking material, the lead may be easily rotated and ultimately extracted by traction. The proximal end of the tip electrode assembly and the distal end of the lead body have complementary interengaging stop surfaces that prevent rotation of the tip electrode assembly during rotation of the special stylet so that rotation of the tip assembly and consequent injury is prevented.
In accordance with one specific exemplary embodiment of the invention, there is provided a body implantable lead assembly adapted to transmit electrical signals between a proximal end portion of the lead assembly and a distal end portion of the lead assembly and to thereby stimulate selected body tissue and/or sense electrical signals therefrom. The lead assembly has a longitudinal axis and comprises a lead body lying along the longitudinal axis, the lead body having a proximal end and a distal end. A rotatable threaded element is carried by the distal end of the lead body. An electrical conductor extends between the proximal end and the distal end of the lead body for transmitting the electrical signals, the conductor having a distal end. A sheath of insulative, biocompatible material encloses the electrical conductor for electrically insulating the electrical conductor from body tissue and body fluids. A tip electrode, disposed at the distal end of the lead body, is electrically connected to the distal end of the electrical conductor, and further has a proximal end including a threaded bore. The rotatable threaded element is screwed into the threaded bore, whereby unscrewing of the rotatable threaded element disconnects the tip electrode from the electrical conductor and disengages the lead body from the tip electrode.
In accordance with another aspect of the invention, the proximal end of the tip electrode and the distal end of the lead body have interengageable surfaces preventing relative rotation between the tip electrode and the lead body during unscrewing of the rotatable threaded element, thereby preventing injury which might result from rotation of an encapsulated tip electrode. The interengageable surfaces preferably comprise complementary, longitudinal surfaces extending radially relative to the longitudinal axis.
In accordance with yet another aspect of the invention, the tip electrode has an active surface including a plurality of concentric ridges coaxial of the longitudinal axis of the lead assembly. Such concentric ridges tend to prevent displacement or microdislodgment of the tip electrode relative to the myocardium. Still further, at least the lead body and the tip electrode of the lead assembly of the present invention are isodiametric thereby facilitating extraction of the lead body once uncoupled from the tip electrode.
In accordance with yet another feature of the present invention, the electric conductor comprises a coiled conductor having a lumen. The rotatable threaded element has a proximal end configured to mate with the distal end of a rotatable stylet insertable into the lumen of the coiled conductor, rotation of the stylet unscrewing the rotatable threaded element.